>>The whole world didn’t collapse during the race riots of the sixties. I don’t see why riots in American cities would in and of themselves produce a worldwide collapse of civilization.<<
The race riots lasted a couple of days (same for the Rodney King riots). It wasn’t “the collapse” — it was a hiccough.
If you are using modern history, you need something a lot more permanent.
I just went back to get a few of the riots from the ‘60s and ‘70s.
1964 - New York City 1964 race riot, July 18–23 (New York City)
1964 - Rochester 1964 race riot, July 24–25 (Rochester, New York)
1964 - Jersey City 1964 race riot,[9] August 2–4 (Jersey City, New Jersey)
1964 - Elizabeth 1964 race riot,[9] August 11–13 (Elizabeth, New Jersey)
1964 - Dixmoor 1964 race riot[9] August 16–17 (Chicago, Illinois)
1964 - Philadelphia 1964 race riot August 28–30
1965 - Watts Riot, August 11-15 1965, (Los Angeles, California)
1966 - Division Street Riots, June 12–14 (Humboldt Park, Chicago)
1966 - Hough Riots, July 1966 (Cleveland, Ohio)
1966 - Benton Harbor Riot, August–September 1966 (Benton Harbor, Michigan)
1966 - Atlanta riot of 1966, Sep. 6 (Atlanta, Georgia)
1966 - Sunset Strip curfew riots (Los Angeles, California)
1967 - Tampa Riots of 1967, June 1967 (Tampa, Florida)
1967 - Buffalo riot of 1967, June 27 (Buffalo, New York)
1967 - Newark riots, July 12–18, 1967 (Newark, New Jersey)
1967 - Plainfield riots, July 14–20, 1967 (Plainfield, New Jersey)
1967 - Cairo riot, July 17 (Cairo, Illinois)
1968 - Washington, D.C. riots, April 1968 (Washington, D.C.)
1968 - Baltimore riot of 1968, April 6–12 (Baltimore, Maryland)
1968 - Chicago riot of 1968 April 7–14 (Chicago, Illinois)
1968 - Kansas City riot of 1968, April 1968 (Kansas City, Missouri)
1968 - Louisville riots of 1968, May 27, (Louisville, Kentucky)
1968 - Glenville Shootout, (Cleveland, Ohio)
1968 - Democratic National Convention riot, August 1968, (Chicago, Illinois,
1969 - Days of Rage, Oct. 1969, (Chicago, Illinois)
1970 - Kent State shootings, May 1970, (Kent, Ohio)
1970 - Hard Hat riot, Wall Street, May 8, 1970, (New York City, New York)
1971 - May Day Protests 1971, May 1971, (Washington, D.C.)
1971 - Camden Riots, August 1971, (Camden, New Jersey)
1974 - Cherry Blossom Festival at the Richmond Stadium, (Richmond, Virginia)
1975 - Livernois-Fenkell riot (Detroit, Michigan)
1977 - New York City Blackout riot, July 1977, (New York City)
1979 - Greensboro Riot/Shootings, Nov. 1979, (Greensboro, North Carolina)
I can’t agree with your statement that widespread rioting in America would necessarily require or produce a worldwide collapse of civilization.